Friction stir welding (FSW) is a relatively new welding process for joining parts of materials such as metals, plastics, and other materials that will soften and commingle under applied frictional heat to become integrally connected. A detailed description of an FSW apparatus and process may be found in Patent Publications WO 93/10935 and WO 95/26254; and U.S. Pat. No. 5,460,317, all of which are herein fully incorporated by reference. One useful apparatus for FSW is shown in FIGS. 1A and 1B and includes a shoulder 14' at its distal end, and a nonconsumable welding pin 16' extending downward centrally from the shoulder. As shown, two parts to be welded together, exemplified by plates 10A' and 10B' on backing plate 12', are aligned so that edges of the plates along the weld joint are in direct contact. As the rotating tool W' is brought into contact with the weld interface between plates 10A' and 10B', the rotating pin 16' is forced into contact with the material of both plates, as shown. The rotation of the pin in the material and rubbing of the shoulder against the upper surface of the material produce a large amount of frictional heating of both the welding tool and the plate interface. This heat softens the material of the plates in the vicinity of the rotating pin and shoulder, and in concert with deformation created by the rotating pin, causes commingling of material, which, upon hardening, forms a weld. The tool is moved longitudinally along the interface between plates 10A' and 10B', thereby forming an elongate weld along the interface between the plates. When the weld is completed, the welding tool is retracted.
FSW has been successfully used for welding aluminum alloys. For welding together flat workpieces, suitable run-on and run-off extensions can be utilized. These extensions, or end-tabs, provide starting and stopping points along a weld seam that may be later trimmed away. The end-tabs serve to transition a FSW tool to and from the workpieces without causing undue disturbance to the weld. A problem arises when welding workpieces that cannot have end-tabs, for example, workpieces with a circumferential geometry such as domes and cylinders, or workpieces with any continuous curved surface. In these situations, the FSW tool must be retracted from the weld at one point. The retraction leaves an exit hole behind on the weld, which remains unfilled and creates a discontinuity in the weld path.
To repair holes in steel, friction hydro pillar processing (FHPP) has been developed. FIG. 2 is a schematic diagram of a prior art FHPP for steel. The process involves rotating a consumable rod 20 coaxially in a generally cylindrical hole 22 to be filled, while under an applied load to generate continuously a plasticized layer 24. The rotating rod 20 heats with friction to generate plasticized layer 24 in an almost hydrostatic condition. Plasticized layer 24 develops at a rate faster than the feed rate of rod 20, causing plasticized layer 24 to rise along hole 22 while leaving beneath a dynamically recrystallized deposit material 26. FHPP, which was developed specifically for steel, however, is not directly applicable to aluminum alloys due to aluminum's reduced high-temperature strength and high oxidation rate behavior. Specifically, while a certain amount of heat is necessary to cause softening of aluminum to fill a void, excessive frictional heat generated during FHPP tends to degrade the structural and metallurgical properties of aluminum alloys. A need exists to provide a method for repairing voids in aluminum alloys, in particular, filling exit holes left from FSW process, with material the same as, or similar in composition to, the parent material in such a manner that the repaired section has structural and metallurgical properties that are the same as, or better than, the parent metal.